1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
An image forming apparatus employing an electrophotographic system (hereinafter referred to simply as “an image forming apparatus”) has been widely used in various fields today on account of its advantage that a high-quality image can be printed on a recording medium through simple operations. The image forming apparatus includes, for example, a photoreceptor drum, a charging section, an exposing section, a developing section, a transferring section, and a fixing section. The photoreceptor drum has a photosensitive layer by which a surface of the photoreceptor drum is formed. The charging section charges the surface of the photoreceptor drum until the surface has predetermined polarity and potential. The exposing section forms an electrostatic latent image on the charged surface of the photoreceptor drum. The developing section supplies a toner to the electrostatic latent image on the surface of the photoreceptor drum so that a toner image is formed. The transferring section transfers the toner image on the surface of the photoreceptor drum onto the recording medium. The fixing section fixes the toner image to the recording medium. By way of processes in these respective sections, an image in accordance with image information is formed on the recording medium.
In the image forming apparatus, control on toner density of the toner image, called a process control, is carried out in order to maintain image quality of the image at high level. A change in the amount of toner electrostatic charge easily occurs over time by influences such as humidity, temperature, and heat developed inside the image forming apparatus. Accordingly, the toner density of the toner image cannot be constant even when the potential of the charged surface of the photoreceptor drum, potential upon exposure for forming the electrostatic image, development bias voltage applied to a developing device, and the like element are respectively set at constant levels. A control for bringing the toner density back to the reference toner density which is established upon designing the image forming apparatus, is called the process control. In the process control, first of all, the development bias voltage is continuously changed, for example, so that a plurality of toner patches (toner images) which are sequentially different in toner density are formed on the surface of the photoreceptor drum. The toner density of each of these toner patches is detected by a toner density detecting section, and a result thus detected is inputted to a control section disposed in the image forming apparatus. In the control section, the detected result is compared with the reference toner density which has been inputted in advance so that one toner patch is selected whose toner density is the closest to the reference toner density, and a value of the development bias voltage applied in forming the toner patch is specified. On the basis of the specified value of the development bias voltage, a toner image is formed on the surface of the photoreceptor drum, thus allowing stable formation of a toner image whose density is almost equal to the reference toner density. In addition, in the process control, the toner density can be also adjusted by changing voltage in charging the photoreceptor drum, potential in exposing the photoreceptor drum, etc.
As described above, the process control is very important for stabilizing the toner density and thus the image density. In the process control, a photoelectric sensor having a light-emitting element and a light-receiving element is mainly used as the toner density detecting section. The light-emitting element emits light to the toner patch. The light-receiving element detects a reflection light amount of light emitted from the light-emitting element to the toner patch, and converts the amount to an electric signal which is then delivered to the control section of the image forming apparatus. The photoelectric sensor detects the toner density almost precisely, therefore being useful. The photoelectric sensor, however, relatively often suffers from malfunctions which may cause difficulty in performing the process control. In the image forming apparatus which adopts the process control, the image forming operation is, in many cases, brought to a halt when the toner density detecting sections such as the photoelectric sensor goes down. In such a case, it is necessary to refrain from using the image forming apparatus until the maintenance check is conducted. Along with more and more demands for enhancement in performance of the image forming apparatus, there has been a demand for an image forming apparatus which is capable of continuing the image forming operation even with the toner density detecting sensor broken and thus forming an image whose quality is substantially equal to that obtained while the toner density detecting sensor properly operates. In response to the above demand, there have been various proposals.
For example, there has been proposed an image forming apparatus which includes a patch density detecting section, a voltage difference detecting section, a reference value determining section, an adjustment value determining section, a memory updating section, an output value readout section, and an average value calculating section (refer to Japanese Unexamined Patent Publication JP-A 6-301257 (1994)). In the image forming apparatus of Japanese Unexamined Patent Publication JP-A 6-301257 (1994), a photoelectric sensor is used for the patch density detecting section. The voltage difference detecting section determines a difference (hereinafter referred to as “a voltage difference”) between a voltage value corresponding to light intensity measured by the toner density detecting section and a voltage value inputted last time. The reference value determining section determines whether or not the voltage difference obtained by the voltage difference detecting section falls in a range of reference value. The adjustment value determining section determines an output adjustment value based on such a determined result given by the reference value determining section that the voltage difference falls in the range of reference value. The memory updating section stores the latest output adjustment value to be inputted to the patch density detecting section, and the output adjustment value. The output value readout section reads out respective output adjustment values which have been obtained past for multiple occasions, based on such a determined result that the voltage difference given by the reference value determining section is out of the range of reference value. The average value calculating section determines an average value of the respective output adjustment values for multiple occasions read out by the output value readout section. In the image forming apparatus of JP-A 6-301257 (1994), when the patch density detecting section 80 properly works, the control is carried out based on the output adjustment value which is determined by the adjustment value determining section. On the other hand, when the patch density detecting section malfunctions and thus outputs an abnormal value that could not be taken under normal conditions, the control is carried out based on the output adjustment value determined by the average value calculating section. The image forming apparatus of JP-A 6-301257 (1994) in which the control is carried out based on the voltage difference, however fails to carry out a proper control in the case where the voltage value outputted by the patch density detecting section gradually changes, since the malfunction of the patch density detecting section is not sufficiently reflected to the adjustment value.